A genetic variant that may explain why some people respond to a common type of antidepressant drugs while others do not has been identified by scientists.

The discovery could eventually lead to the development of genetic tests that can provide patients who are suffering from depression with tailored prescriptions that are more likely to work for them.

The findings, from a US team, also suggest a possible reason for the mixed research evidence for the effectiveness of selective serotonin reuptake inhibitors (SSRIs), which include Prozac and Seroxat. SSRIs account for most of the 31 million antidepressant prescriptions in Britain each year. The drugs, however, remain stubbornly ineffective for many people. Research has shown that they are only slightly more effective than inert sugar pills across the population as a whole, and many of their benefits are thought to derive from the placebo effect.

There is also growing concern that some SSRIs may be linked to an increased risk of suicidal thoughts, particularly among adolescents.

The research, led by Roy Perlis, of Massachusetts General Hospital, Boston, suggests that individuals’ genetic profiles may at least in part influence the different effects that the drugs have on different patients. The findings could indicate that the variable picture of effectiveness occurs because only some people are genetically amenable to SSRIs.

The small benefit of the drug over a placebo, seen in many trials, could be the result of a masking effect caused by the inclusion of patients whose genes make them immune to SSRIs. The lack of effect on these patients may be obscuring a much greater benefit among those who are able to respond to the drugs.

In the longer term, further findings about the genes that govern people’s response to SSRIs could lead to tests that can determine whether they are likely to work for particular patients.

Those whose genes mean that the drugs will probably be ineffective could then avoid taking medication from which they will not benefit, yet which can have side-effects including impotence, loss of libido, nausea and, in rare cases, suicidal thoughts.

The study is published in the journal Neuropsychopharmacology.

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